Soil Particulate Carbon as Indicator of Management Practices in No-Tillage System.

The capability to detect management effects for sustained production in no-tillage system depends on knowledge of how different soil organic fractions are impacted by cropping systems and how they are related to carbon storage in the soil. The purpose of this study was to evaluate how crop sequences influence soil total carbon, soil carbon associated to   soil aggregates (particulate C)  and soil carbon associated to soil minerals, in a Rhodic Eutrudox, under no-tillage system since 2002, at  Jaboticabal County – Sao Paulo State, Brazil (21°14′S, 48°17′W). The crop sequences consisted of three summer crops (corn monocrop, soybean monocrop and soybean/corn rotation) combined with seven winter crops (corn, sunflower, oilseed radish, millet, pigeon pea, grain sorghum and sunn hemp). In October 2008, after the winter crops, soil samples were taken in the 0‑10 cm depth. Total soil organic carbon (TOC) was determined in the bulk soil sample and in both fraction of soil samples: >53 µm (particulate C - POC) and <53 µm (C associated to soil mineral). The soybean/corn rotation showed lowest TOC and POC contents when compared to corn and soybean monocrop. In the soybean/corn rotation was observed higher microbiological activity, which explains the lower TOC soil contents due to the utilization of the carbon compounds by the microorganisms as a source of energy. The soybean monocrop showed the highest POC (35%), due to the soybean residues lower C/N ratio and its fast decomposition. The winter crops did not affect the TOC contents, but the small amount of sunflower residues provided lower POC contents when compared to those produced by the grain sorghum. Higher differences between crop sequences were observed when soil particulate carbon was determined, which indicates higher sensitivity for detecting management practices when this fraction of carbon is determined in the soil.